A common form of solar cell module or photovoltaic (PV) module is made by interconnecting individually formed and separate solar cells, e.g., crystalline silicon solar cell, and then mechanically supporting and protecting the cells against environmental degradation by integrating the cells into a laminated solar cell module. The laminated modules usually comprise a stiff transparent protective front panel or sheet, and a rear panel or sheet typically called a “backsheet” or “backskin”. Interconnected solar cells and an encapsulant are disposed between the front and back sheets so as to form a sandwich arrangement. A necessary requirement of the encapsulant (or at least that portion thereof that extends between the front sides of the cells and the transparent front panel) is that it be transparent to solar radiation. The typical mode of forming the laminated module is to assemble a sandwich comprising in order a transparent panel, e.g., a front panel made of glass or a transparent polymer, a front layer of at least one sheet of encapsulant, an array of solar cells interconnected by electrical conductors (with the front sides of the cells facing the transparent panel), a back layer of at least one sheet of encapsulant, a sheet of scrim to facilitate gas removal during the lamination process, and a backsheet or back panel, and then bonding those components together under heat and pressure using a vacuum-type laminator. The back layer of encapsulant may be transparent or any other color, and prior art modules have been formed using a backsheet consisting of a thermoplastic polymer, glass or some other material.
Although the lamination process seals the several layered components together throughout the full expanse of the module, it is common practice to apply a protective polymeric edge sealant to the module so as to assure that moisture will not penetrate the edge portion of the module. The polymeric edge sealant may be in the form of a strip of tape or a caulking-type compound. Another common practice is to provide the module with a perimeter frame, usually made of a metal like aluminum, to provide mechanical edge protection. Those techniques are disclosed or suggested in U.S. Pat. No. 5,741,370. That patent also discloses the concept of eliminating the back layer of encapsulant and bonding a thermoplastic backskin directly to the interconnected solar cells.
A large number of materials have been used or considered for use as the encapsulant in modules made up of individual silicon solar cells. Until at least around 1995, ethylene vinyl acetate copolymer (commonly known as “EVA”) was considered the best encapsulant for modules comprising crystalline silicon solar cells. However, EVA has certain limitations: (1) it decomposes under sunlight, with the result that it discolors and gets progressively darker, and (2) its decomposition releases acetic acid which in turn promotes further degradation, particularly in the presence of oxygen and/or heat.
U.S. Pat. No. 5,478,402 discloses use of an ionomer as a cell encapsulant substitute for EVA. The use of ionomer as an encapsulant is further disclosed in U.S. Pat. No. 5,741,370. Ionomers are acid copolymers in which a portion of the carboxylic acid groups in the copolymer are neutralized to salts containing metal ions. U.S. Pat. No. 3,264,272 discloses a composition comprising a random copolymer of copolymerized units of an alpha-olefin having from two to ten carbon atoms, an alpha, beta-ethylenically-unsaturated carboxylic acid having from three to eight carbon atoms in which 10 to 90 percent of the acid groups are neutralized to salts with metal ions from Groups I, II, or III of the Periodic Table, notably, sodium, zinc, lithium, or magnesium, and an optional third mono-ethylenically unsaturated comonomer such as methyl methacrylate or butyl acrylate.
It is known to use a rear panel or backsheet that is made of the same material as the front panel, but a preferred and common practice is to make it of a different material, preferably a material that weighs substantially less than glass, such as a polyvinyl fluoride polymer available under the tradename Tedlar® from E.I. Du Pont de Nemours Co. (DuPont). A widely used backsheet material is a Tedlar®/polyester/ethylene vinyl acetate laminate. Another common backsheet uses a trilayer structure of Tedlar®/Polyester/Tedlar®, also called TPT™, described in WO 94/22172. This structure allows the fluoropolymer to protect both sides of the polyester from photo-degradation. However, Tedlar® and Tedlar® laminates are not totally impervious to moisture, and as a consequence over time the power output and/or the useful life of modules made with this kind of backsheet material is reduced due to electrical shorting resulting from absorbed moisture.
Due to the price and the supply concern, the PV industry has been gradually evaluating new alternatives, such as backsheets derived from PET, polyamides, etc. For example, WO 2008/138021 discloses PV modules with backsheets based on polyamides derived from linear and/or branched aliphatic and/or cycloaliphatic monomers, which have an average of at least 8 and most 17 carbon atoms, such as nylon 12. However, polyamides are semi-crystalline polymers with high degree of crystallinity, which can lead to brittleness, low flexibility and excessive shrinkage. High moisture absorption is especially a problem for nylon-6 and nylon-66, the most inexpensive polyamides. Water absorption causes dimensional instability, poor weatherability, and, most importantly, reduces insulation capability. While nylon-11 and nylon-12 have better moisture resistance and weatherability, the melting temperature may be too low for use in some lamination processes of PV module assembly.
U.S. Pat. No. 5,741,370 discloses using as the backskin material a thermoplastic olefin comprising a combination of two different ionomers, e.g., a sodium ionomer and a zinc second ionomer, with that combination being described as producing a synergistic effect which improves the water vapor barrier property of the backskin material over and above the barrier property of either of the individual ionomer components. The patent also discloses use of an ionomer encapsulant with the dual ionomer backskin.
It is known that thermoplastic blends or alloys based on ionomers and polyamides have a combination of desirable properties (see U.S. Pat. Nos. 4,174,358, 5,688,868, 5,866,658, 6,399,684, 6,569,947, 6,756,443 and 7,144,938, 7,592,056, 8,057,910, 8,062,757 and 8,119,235). For example, U.S. Pat. No. 5,866,658 discloses a blend of an ionomer dispersed in a continuous or co-continuous polyamide phase in the range of 60/40 weight % to 40/60 weight % used for molded parts exhibiting toughness, high gloss, abrasion/scratch resistance, and high temperature properties. U.S. Pat. No. 6,399,684 discloses similar blends also containing phosphorous salts such as a hypophosphite salt. See also U.S. Patent Applications 2002/0055006, 2005/007462, 2006/0142489, 2008/0161503, 2009/0298372, 2013/0167966, 2013/0171390, 2013/0171394, 2013/0172470 and 2013/0172488.
U.S. Pat. Nos. 5,700,890, 5,859,137, 7,267,884 and U.S. Patent Application Publications 2005/0020762A1, and 2006/0142489A1 disclose polyamides toughened with ionomers of ethylene copolymers containing a monocarboxylic acid and a dicarboxylic acid or derivative thereof. U.S. Patent Application Publication 2011/0020573 discloses a blend comprising a polyamide, an ionomer of an ethylene copolymer containing a monocarboxylic acid and a dicarboxylic acid or derivative thereof, and a sulfonamide. U.S. Pat. No. 8,586,663 discloses a blend comprising a polyamide, an ionomer of an ethylene copolymer containing a monocarboxylic acid and a dicarboxylic acid or derivative thereof, and a second ionomer. U.S. Pat. No. 7,592,056 discloses blends of polyamides with mixed ion ionomers, including zinc and sodium mixtures.
U.S. Pat. No. 6,660,930 discloses photovoltaic modules comprising backskins comprising a nylon/ionomer alloy.
Photovoltaic modules can be assessed for moisture permeation and weatherability by cyclic treatment with high moisture and temperature and cold temperature in standardized “stress tests”. It is desirable to provide PV modules that are capable of withstanding such stress tests for substantially more than 1000 hours. Thus, it also is desirable to provide backsheet materials that provide PV modules that are capable of withstanding such stress tests.